The present invention relates to a method for the suspension polymerization of vinyl chloride monomer. More particularly, the invention relates to a method for the suspension polymerization of vinyl chloride monomer capable of providing a vinyl chloride resin having advantageous properties such as high plasticizer absorption and small content of fish-eyes with a decreased amount of polymer scale deposition on the reactor walls.
Vinyl chloride polymers are heretofore produced mostly by the method of so-called suspension polymerization of vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride in combination with a minor amount of other ethylenically unsaturated monomers in an aqueous suspension medium containing a dispersing or suspending agent, such as polyvinyl alcohol, hereinafter referred to as PVA, water-soluble cellulose derivatives and the like, of which PVAs are the most conventional and widely used.
The conventional procedure for the suspension polymerization of vinyl chloride is as follows. Thus, water as the suspension medium and a PVA, usually in the form of an aqueous solution, are first introduced into the polymerization reactor and, after evacuation to exclude the atmospheric air from the reactor, the monomer or monomer mixture is introduced into the reactor under agitation and, when a uniform dispersion of the monomer droplets in the aqueous medium has been established, a polymerization initiator, which is usually soluble in the monomer phase, is added to the polymerization mixture in the reactor to start the polymerization reaction. A problem in this conventional procedure is that, because of the relatively lengthy contacting time of the vinyl chloride monomer droplets and the PVA as the suspending agent before start of the polymerization reaction, a considerable portion of the PVA is adsorbed on the monomer droplets resulting in a decrease in the effective concentration of the suspending agent in the aqueous medium so as to decrease the stability of the monomer suspension.
As a consequence, the polyvinyl chloride resin obtained by this conventional procedure has several problems in the properties such as broad particle size distribution and low porosity responsible for the slow rate of plasticizer absorption to decrease the productivity in the molding processes and retarded gelation to cause non-uniform quality of the molded articles. The number of fish-eyes is also considerably large to result in poor outer appearance of the molded articles. Apart from the quality of the molded articles of the resin powder, the low porosity of the resin particles is an unfavorable condition for the removal of the unreacted monomer adsorbed in the resin particles from which the monomer vapor is subsequently emitted in the course of the resin processing and molding process to cause pollution of the working environments with a serious problem against the workers' health. When the molded articles of the polyvinyl chloride resin contain the unreacted vinyl chloride monomer, there may be caused a serious problem on the public health.
In view of the above described situations, solutions for the problems are sought by attempting use of various types of PVAs to be used as the suspending agent. For example, so-called oil-soluble PVAs having a low degree of polymerization and low degree of saponification as well as modified PVAs having various kinds of substituent groups have been proposed as a suspending agent. The oil-soluble PVAs as the suspending agent have a disadvantage that the bulk density of the resin powder produced therewith can not always be high enough although they are effective in improving the resin powder in respect of the releasability of the unreacted monomer and decrease in the number of fish-eyes. Modified PVAs having various substituent groups are disadvantageous in respect of the stability of the monomer suspension in the aqueous medium resulting in a relatively large amount of polymer scale deposition on the reactor walls and broadness of the particle size distribution of the resin powder.